and \(\beta ^a_i\)  is the power of \(1-x\)  with which the forward limit of the GPD \(K_i^a\)  vanishes for \(x \rightarrow 1\) .
Eq. (REF ) is a generalization of the famous Drell-Yan relation [1]}.
The phenomenological values of the powers \(d_i^a\)  are listed in Tab. REF . Note the differences
between the powers of \(u\)  and \(d\) -quarks. This implies that at very large \(-t\)  only \(u\) -quark flavor form factors
contribute. In case of the helicity non-flip GPDs the powers \(\beta _i^u\)  are slightly larger than expected
according to perturbative QCD arguments [2]}, [3]}.
We however stress that, in practice, the powers, \(\beta ^a_i\) , are fixed in a region of \(x\)  less than about
0.8 for the helicity non-flip GPDs, and even a smaller \(x\) -region for the transversity ones.
For larger \(x\)  there is no experimental information on the forward limits available at present. Therefore, the
powers \(\beta ^a_i\)  are to be considered as effective powers which are likely subject to
changes as soon as data at larger \(x\)  become available. These powers affect the energy dependence of the
cross sections. For photoproduction the energy dependence of the cross section at fixed \(\cos {\theta }\) 
can readily be read off from (REF ) and the subprocess amplitudes discussed in Sect. . One finds the familiar \(1/s^7\)  scaling behavior of the cross section at fixed
\(\cos {\theta }\)  for the twist-2 contribution
and \(1/s^8\)  for the twist-3 one provided the form factors, including the prefactors of \(\sqrt{-t}\)  and \(t\) 
appearing in (REF ), drop as \(1/t^2\) . Deviations from that behavior change the scaling behavior as
do the logs from \(\alpha _s\)  and the evolution of the DA parameters. As can be seen from Tab. REF 
our form factors increase the power of \(s\)  with which the cross section falls. In the energy range we explor
the effective scaling is \(1/s^9\) .
